git-gui: Change prior tree SHA-1 verification to use git_read
git-gui: Change prior tree SHA-1 verification to use git_read

This cat-file was done on maint, where we did not have git_read
available to us.  But here on master we do, so we should make
use of it.

Signed-off-by: Shawn O. Pearce <f01032651c12a22b2103346ba96b3b6b231f9d3c@spearce.org>
destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git